I'm sure Bax will have something to say about people like myself but I am one of the really bad ones when it comes to breaking in an engine. I do it with the SK line of engines but I'm a bit hit and miss with the other brands I use, OS and YS. Sometimes I do and sometimes I don't. When I first started flying RC I didn't bother at all and over time I sometimes do. I don't spend a lot of time worrying about it when I buy a new car, I just fire it up and drive. I do keep a close eye on my oil and water the first few thousand miles. I used to build VW engines for both street and sand drags. Street engines I did give the owner instructions for the first 500 miles but for off road use?? For a cheap {not so cheap} thrill go take a new engine to the dyno shop!! Be ready to throw away the pants you are wearing. Brand new engine, never fired up and put through a test that will scare the peewaddle out of you. When it's over you know everything there is to know about the engine but it's not for the faint of hart. I would rather be breaking in my glow engines while in the air then burning fuel on the ground. I do a lot of loops and stall turns the first few flights, gets them hot on the way up and cools them on the way down. I just haven't ever noticed a nickles worth of difference between an engine I have broken in on the graound or one I broke in while flying. I do run new engines a bit on the fat side but not for long. I used to work in a small engine repair shop and sold brush cutters and chain saws, no one ever wastes time breaking in one of those, just fire it up and go cut a couple of cords. Perhaps someone like Bax could give a more detailed reason for the break in that the instructions say to do. Is it really something that has to be done?? I have built engines that were on the stand one day and in a car and on a race track the next day so I do question it.